From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9C6684AC158; Tue, 15 Sep 2026 19:00:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789498821; cv=none; b=FDnaBcsFlabd+XjBfgNOIYpWDjq4fsMeJcCcRdIhS3FE9sFf3sgcUQKBMyISt/3VNFJPj7U8TcFTBm6JNFwksv4l2lHKtlVjZKI16YaJpZU1fp/S19Kw/HR2ZGY1GhHMjoWIkVCCr+koSVaE0jHp6A00snK8EFN8pwY2KDRYlis= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789498821; c=relaxed/simple; bh=/pxPtBDRL/rubNz3T85ru1lxCUdEgL1csnJGWfVKvsw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=GqmGZ+D+b0m16WrZ+LAU4nCNq7ZeqY8/MkYDWu7U8ZexwCzwKZMJHnBobpJLv1Y7OedjvbrLV4gt3JE5DLSw1kPRr7qVLfc/fQ1o+XSlxmD98yWl+6iUXp7iCHF8kKIPpRvIieZi+ah2VKfvPPY2DtWTTyfIVZV3L5Jv22r+pR0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=j0iSBaet; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="j0iSBaet" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4C7C11F00899; Tue, 15 Sep 2026 19:00:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789498819; bh=bH9D6fx46WAVM8PGx4wEKkSYBjKGmw5AvziupNUvWGk=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=j0iSBaetC/3YCi18xNuSeB+n+7VnOF+QOgAvJx7ktLSNyWzdoyXHzZ/YPBs5rtm+n WA6gaCeYDOM7G68qSWfRsHvbbAN1voKYHzwbAYckQsRPEBWlfdZePBmdFPmQ1OMU/N sUCxn5+r/cR7FN5oCIcXoHFsLztawdQ7lZ2pVlHqL5qvTKh/9Qqc0Ta8A275Irt74k zf/eAGrgknulaZr5ylmpMNiuL5cL1xjelXuos4e0Xj/d9MPGQhh5E4WSCnRmy8X7rs HaD3mC4eIWM8qko0PgiUDZhDxobQu7+Yi4Weeos/5iSyNoUm6TMJqr9E5mfS6sKjxo y4BshIH81fNpw== Date: Tue, 15 Sep 2026 12:00:19 -0700 From: Kees Cook To: Bill Wendling Cc: linux-kernel@vger.kernel.org, codemender-patching+linux@google.com, Russell King , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , "James E.J. Bottomley" , Helge Deller , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Ian Abbott , H Hartley Sweeten , Tony Luck , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Matthew Brost , Thomas =?iso-8859-1?Q?Hellstr=F6m?= , Rodrigo Vivi , Dmitry Torokhov , Matthias Schwarzott , Mauro Carvalho Chehab , Tony Nguyen , Przemek Kitszel , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Arend van Spriel , Rob Herring , Saravana Kannan , Krzysztof Kozlowski , Sylwester Nawrocki , Peter Griffin , Alim Akhtar , Linus Walleij , Anil Gurumurthy , Sudarsana Kalluru , "Martin K. Petersen" , Trond Myklebust , Anna Schumaker , Mike Marshall , Martin Brandenburg , Jiri Pirko , Simon Horman , Chuck Lever , Jeff Layton , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Jaroslav Kysela , Takashi Iwai , Andrew Morton , "Mike Rapoport (Microsoft)" , Kanglong Wang , Tiezhu Yang , Qiang Ma , Randy Dunlap , Pengpeng Hou , Ard Biesheuvel , Breno Leitao , Thorsten Blum , Harshit Mogalapalli , Greg Kroah-Hartman , Lyude Paul , Ashutosh Desai , Imre Deak , Dmitry Baryshkov , Johan Hovold , Johannes Berg , Miri Korenblit , Alexander Stein , Cryolitia PukNgae , Jiaming Zhang , Will Porter , Cen Zhang , =?iso-8859-1?Q?C=E1ssio?= Gabriel , Rong Zhang , Arun Raghavan , linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linux-edac@vger.kernel.org, dri-devel@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-input@vger.kernel.org, linux-media@vger.kernel.org, intel-wired-lan@lists.osuosl.org, netdev@vger.kernel.org, linux-wireless@vger.kernel.org, brcm80211@lists.linux.dev, brcm80211-dev-list.pdl@broadcom.com, devicetree@vger.kernel.org, linux-samsung-soc@vger.kernel.org, linux-gpio@vger.kernel.org, linux-scsi@vger.kernel.org, linux-nfs@vger.kernel.org, devel@lists.orangefs.org, linux-hardening@vger.kernel.org, linux-sound@vger.kernel.org Subject: Re: [PATCH 03/28] ARM: atags: Replace strlcat() with snprintf() for cmdline extend Message-ID: <202609151155.CC22827@keescook> References: <20260915081910.4142719-1-morbo@google.com> <20260915081910.4142719-4-morbo@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260915081910.4142719-4-morbo@google.com> On Tue, Sep 15, 2026 at 08:18:20AM +0000, Bill Wendling wrote: > --- a/arch/arm/kernel/atags_parse.c > +++ b/arch/arm/kernel/atags_parse.c > @@ -121,9 +121,10 @@ __tagtable(ATAG_REVISION, parse_tag_revision); > static int __init parse_tag_cmdline(const struct tag *tag) > { > #if defined(CONFIG_CMDLINE_EXTEND) > - strlcat(default_command_line, " ", COMMAND_LINE_SIZE); > - strlcat(default_command_line, tag->u.cmdline.cmdline, > - COMMAND_LINE_SIZE); > + size_t len = strlen(default_command_line); > + > + snprintf(default_command_line + len, COMMAND_LINE_SIZE - len, > + " %s", tag->u.cmdline.cmdline); > #elif defined(CONFIG_CMDLINE_FORCE) > pr_warn("Ignoring tag cmdline (using the default kernel command line)\n"); > #else It's pretty clear we have a pattern of "simple append" that is needed, and while strlcat() does that, it's horrible. I feel like we need an appending scnprintf(), and it needs to return like strscpy() does (i.e. -E2BIG on truncation). My goal would be: - don't open-code string length math - never leave the destination unterminated - allow for format strings - yes/no return indication for "did this truncate?" Initializing a whole seq_buf struct for these small cases is too much overhead... For both cases (small without seq_buf, large with seq_buf), it is also frequently needed to know the resulting strlen. seq_buf doesn't do this right today, and a strscpy-style return value also doesn't tell us. :( -- Kees Cook